The present invention relates to a lithographic technique for transferring a precise pattern such as an integrated circuit pattern. More particularly, the present invention pertains to a pattern transfer mask which enables alignment between a wafer and the mask to be effected at high precision, the pattern transfer mask being applicable to various kinds of lithographic technique such as photolithography and X-ray lithography.
Alignment between a wafer and a mask in a pattern transfer operation has heretofore been effected in such a manner that, as exemplarily shown in FIGS. 1a and 1b, target marks for alignment 5 and 6 provided on a wafer 4 are illuminated with light 9 for alignment, and the reflected light is detected. In this case, the reflected light shows changes in intensity as those shown in FIG. 1b. In the figure: the reference numerals 5' and 5" represent the intensities of reflected light from the target mark 5; 6', the intensity of reflected light from the target mark 6, the reflected light intensity 6' being lowered because the illumination light is scattered by the target mark 6 having a rough surface; 11', the intensity of reflected light 11 from a radiation absorber 2 formed on the mask; and C, a slice level of either the reflected light intensity 5' or 5" from the target mark 5. When the intensity of reflected light from the target mark 5 is at a relatively high level as at 5', alignment can be effected precisely. However, in some processes, the intensity of reflected light from the target mark 5 may be reduced to a level lower than the slice level C as in the case of 5", which means that satisfactory alignment cannot be effected. In such case, even when the amplification factor of the detector circuit is increased, the ratio between 5" and 6' is so small that it is impossible to carry out stable and effective alignment. In this conventional method, further, the curves of the actually measured reflected light intensity 11' represent mask patterns which are asymmetric with each other as shown in FIG. 1c due to, for example, possible aberration which may occur in the illumination optics and the detection optics. In consequence, if the slice level is changed from C to C' , the center position of each of the mask patterns is undesirably shifted by 8, and this is erroneously measured. In addition, the above-described conventional method requires a wafer process step of forming the target mark 6 having a rough surface, which fact leads to a rise in costs. If the reflection factor of the target mark 6 is relatively high, when the reflected light intensity is relatively high as in the aluminum layer forming step the reflected light intensity 6' may reach the same level as that of the intensity 11' of reflected light 11 from the absorber 2, thus making it difficult to effect stable and precise alignment.
These facts are well known, particularly, in semiconductor integrated circuit manufacturing processes using X-ray lithography technique.